Known electric linear motion actuators of this type include a ball screw mechanism or a ball-ramp mechanism as a motion converter mechanism for converting the rotary motion of an electric motor to a linear motion.
A ball screw mechanism or a ball-ramp mechanism can increase power to a certain extent when power is transmitted through threads having a lead angle or inclined cam surfaces, of the motion converter mechanism. But these conventional mechanisms cannot increase power to such an extent as required in e.g. an electric disc brake assembly.
JP Patent Publication 6-327190A discloses an electric linear motion actuator including one of the above-mentioned motion converter mechanisms and further including planetary gear speed reduction mechanism to increase the driving force. The addition of such a speed reduction mechanism tends to increase the size of the electric linear motion actuator, making it difficult to compactly design the electric linear motion actuator.
In order to avoid these problems, the inventors of the present application have proposed, in JP Patent Publication 2007-32717A, an electric linear motion actuator which ensures a sufficiently large power increase function without the need to mount a speed reduction mechanism, which is relatively small in linear motion stroke, and which is suitable for use in an electric disc brake assembly.
The electric linear motion actuator disclosed in JP Patent Publication 2007-32717A comprises a housing, an outer ring member mounted in the housing and formed with a helical rib on a radially inner surface of the outer ring member, a rotary shaft mounted coaxially with the outer ring member and configured to be rotated by an electric motor, a carrier supported by the rotary shaft so as to be rotatable about the rotary shaft, and a plurality of planetary rollers mounted between a radially outer surface of the rotary shaft and the radially inner surface of the outer ring member and rotatably supported by the carrier, the planetary rollers being in frictional contact with the rotary shaft and each having a plurality of circumferential grooves arranged with the same pitch as the pitch of the helical rib and meshing with the helical rib, the linear motion actuator being configured such that when the rotary shaft rotates, the planetary rollers revolve around the rotary shaft while rotating about axes of the respective planetary rollers due to frictional contact between the planetary rollers and the rotary shaft, and the carrier is moved linearly in an axial direction of the rotary shaft due to meshing engagement between the helical rib and the circumferential grooves, thereby linearly driving a driven member.
The inventors of the present application also proposed, in JP Patent Application 2008-233380, an electric linear motion actuator of which the carrier is axially fixed in position and the outer ring member is used as the output member to linearly drive a driven member.
The electric linear motion actuator disclosed in JP Patent Publication 2007-32717A, of which the carrier is linearly moved as the output member, includes thrust bearings which are disposed in front of the respective planetary rollers with respect to the direction in which the carrier is advanced to linearly drive the driven member, for supporting the rotation of the respective planetary rollers when the planetary rollers are moved linearly together with the carrier.
The electric linear motion actuator disclosed in JP Patent Application 2008-233380, of which the outer ring member is linearly moved as the output member, includes thrust bearings which are disposed in the rear of the respective planetary rollers with respect to the direction in which the outer ring member is advanced to linearly drive the driven member, for supporting the rotation of the respective planetary rollers when the planetary rollers are moved linearly together with the carrier.
Many of vehicle brake assemblies are hydraulic types. But since the introduction of sophisticated brake control systems such as anti-lock brake systems (ABS), electronic disc brake assemblies, which can carry out such brake control without using complicated hydraulic circuits, are gathering attention in recent years.
An electric disc brake assembly disclosed in JP Patent Publication 2007-32717A includes an electric linear motion actuator of the above-described type which is mounted in a caliper body, and an electric motor. When the electric motor is energized in response to a signal indicating e.g. that the brake pedal has been depressed, a brake member as the driven member is pressed against a braked member by the electric motor through the linear motion actuator, thereby braking the braked member.
The electric linear motion actuators disclosed in JP Patent Publication 2007-32717A and JP Patent Application 2008-233380 are both compact in size and ensure a large power increasing function without mounting a separate speed reducer mechanism. While the planetary rollers revolve around the rotary shaft and simultaneously rotate about their respective axes while being supported by the carrier with the helical rib of the outer ring member engaged in the circumferential grooves of the respective planetary rollers, the respective planetary rollers tend to slightly axially shift from one another. This causes external thrust loads applied to the carrier or the outer ring member as the output member to be distributed unevenly to the respective planetary rollers through the respective thrust bearings, which support the rotation of the respective planetary rollers. This may shorten the life of at least some of the planetary rollers.
An object of the present invention is to uniformly distribute external thrust loads to the respective planetary rollers while the planetary rollers are revolving around the rotary shaft while rotating about their respective axes with the helical rib of the outer ring member in engagement with the circumferential grooves of the respective planetary rollers.